Treatment of waste sulfate liquor



March 20, 1956 E. B. MANCKE TREATMENT OF WASTE SULFATE LIQUOR Filed Jan. 6, 1955 Increasizy Tligygfaiujg Incre Presswuz l' ne INVENTOR United States Patent TREATMENT OF WASTE SULFATE LIQUOR Edgar B. Mancke, Bethlehem, Pa., assignor to Bethlehem Steel Company, a corporation of Pennsyivania Application January 6, 1953, Serial No. 32,884

4 Claims. (Cl. 23--172) This application is a continuation in part of application Serial No. 22,398 filed April 21, 1948, now abandoned.

This invention relates to a process of regenerating waste sulfate liquor resulting from the cleaning of metals, or from the leaching of ores. More particularly, this invention is one in which sulfuric acid, present in waste sulfate pickle liquor, or waste leach liquor, is regenerated by treatment of the waste liquor ina pressure vessel at elevated temperature.

The principal object .of this invention is to effect the rapid separation of free sulfuric acidfrom part of the metal salts found in waste sulfate liquor, thus permitting re-use of the separated acid solution for pickling, leaching, etc.

Another object is to convert the soluble-iron, found in waste sulfate liquor, into an insoluble form which can eaf ly d s ar During the course of pickling of metals with acid, the metal salt, formed in the pickling bath through attack of the metal oxide, or scale, by the acid, tends to build up to a point where it hasa definite retarding action on the pickling rate of the, acid, even when the acid is present in relatively high percentages. While the retarding action of the metal salt during the salt build-up is a gradual one, and the rate of action differs for different types of metal surface treated, some fairly definite limits have been set for allowable metal salt content, beyond which point the time of pickling is of such duration that it is considered uneconomical. It is true, however, that a mineral acid solution containing a low amount of metal salt, say less than or 11 per cent of such salt by weight, and a relatively high acid content (6 per cent or more by weight), can in many cases be used effectively as a pickling medium until the metal content builds up to the pre determined amount. Thus, a waste pickle liquor containing a relatively large quantity of acid, i. e., 6 to per cent by weight, and a metal salt content of 17 or 18 per cent by weight, can be treated .so asv to remove part of the metal, and in this way restore the acid solution, for reuse as a pickling solution. If the acid content of the reclaimed solution is less than normally required for a particular pickling operation, fresh acid may be added to it to produce an acid solution which will attack the scale at a 'suflicient rate of speed.

In the pickling of ferrous metals, sulfuric acid is. the treating acid commonly used. The resultant waste pickle liquor then contains varying amounts of sulfuric acid, along with ferrous sulfate in solution, the ferrous sulfate having been formed by'the action of the acid on the ferrous scale present on the treated material.

Heretofore, the principal method of reclaiming waste sulfate pickle liquor has been that of evaporation of the liquor to a point Where concentration of the sulfuric acid causes a certain percentage of theferrous'sulfate in solution to crystallize, the crystals then being separated from the solution, and the solution, which still contains some on, s. ready o li -u e as a pick ng med m. E aporaice tion processes, such as this one, are both costly andtime consuming. Furthermore, the crystallized iron salt is not suitable for ready disposal, but must be subsequently treated.

My process comprises the operation of subjecting aqueous solutions of iron sulfate and free sulfuric acid containing not less than 7 per cent by weight of ferrous sulfate and free sulfuric acid between 5 per cent and 40 per cent by weight to treatment in an autoclave at high temperatures and pressures in the presence of an oxidizing agent to precipitate a substantial part of the iron, leaving the free sulfuric acid in solution.

In my process, the total pressure employed in the autociave should be at least 250 pounds per square inch and the temperature should be at least 350 F. Under these conditions iron is precipitated as basic iron sulfate. At the completion of the autoclaving operation the solution, containing the free acid, is separated from the precipitated iron compound.

The filtered, or reclaimed, sulfuric acid will, contain some iron in solution. It should be mentioned here that in this invention it is not necessary to convert all of the iron to basic iron sulfate, for a limited amount of dissolved iron in the reclaimed sulfuric acid, although it may retard pickling somewhat, can be tolerated.

i shall now give examples of how my process may be carried out.

Example I A waste pickle liquor containing 10 per cent free sulfuric acid by weight and 13.3 per cent ferrous sulfate by weight is. treated in an autoclave for 1 hour at 420 F. Air is he at osphere above the bathand the. total pressure is maintained at about 480 p. s. i. At the end of the treatment about 90 per cent of the iron is precipitated as a readily filtrable basic iron sulfate.

Example 2 A waste pickle liquor, containing 15.7 per cent ferrous sulfate by weight and 7.9 per cent free sulfuric acid by weight is treated in an autoclave for 1 hour at a temperature of 422 F., air being used as the oxidizing agent, the total pressure being 500 p. s. i. About per cent of the iron is precipitated.

Example 3 A waste pickle liquor, containing 15.7 'per cent FeSO4 and 22.7 per cent of free sulfuric acid by Weight is treated in an autoclave for 1 hour at a temperature of 408 F. Oxygen is used as the oxidizing agent, and the total pressure is 500 p. s. i. About 74.6 per cent of the iron is precipitated.

Example 4 A waste pickle liquor containing 15.7 per cent ferrous sulfate and 7.9 per cent free sulfuric acid, both by weight, is treated in an autoclave for 60 minutes at a temperature of 385 F. Oxygen is used as the oxidizing agent. The total pressure is 250 p. s. i. About three-fourths of the iron is precipitated.

Example 5 Example 6 A, waste pickle liquor containing 7.5 per cent ferrous sulfate by Weight and 25- per cent free sulfuric acid. by weight is treated in the autoclave at a temperature. of

430 F. The oxidizing atmosphere is oxygen-enriched air, the content of oxygen being twice that of the amount of oxygen contained in air. The total pressure was maintained at 500 p. s. i. At the end of 60 minutes about 50 per cent of the iron is precipitated.

As indicated above the temperature of the autoelaving operation should not be less than 350 F. and the total pressure should not be less than 250 pounds per square inch. As the specified examples, given above, indicate, the temperatures and pressures may vary considerably, providing neither temperature nor pressure is below the respective minimum figures of 350 F. and 250 pounds per square inch.

In general, the higher the acid content of the liquor the higher the temperatures or pressures, or both, which should be used. In general, also, the higher the temperature or pressures, or both, the other conditions being the same, the higher the percentage of iron which is precipitated.

The length of time of operation is a factor in the amount of precipitation accomplished. To demonstrate the effect of time on the rate of precipitation of iron in an autoclaving system, a sulfate liquor, containing per cent free acid by weight and 13.3 per cent ferrous sulfate, was treated at 420 F. and 500 p. s. i., using air as an oxidizing agent. The degrees of precipitation with different times are shown in the tabular data given below:

Percent of iron (Fe) precipitated (Percent, average of several runs) 'Iime (Minutes) or by plotting pressure against per cent of dissolved iron precipitated, all other variables being held constant.

Figure 2 is a general curve formed by plotting time against per cent of dissolved iron precipitated, all other variables being held constant.

The general curve shown in Fig. l is a composite of a number of curves formed by plotting temperature against per cent of dissolved iron precipitated, or by plotting pressure against per cent of dissolved iron precipitated, the curves for temperature and those for pressure having substantially the same contour. The curve indicates that in the treatment of a waste sulfate liquor,

in a closed vessel under conditions of temperature and pressure which will induce precipitation of an iron salt, if a constant pressure is maintained, an increase in temperature will give an increasing yield of iron precipitate up to 90 per cent or more of the iron originally present in the treated liquor; likewise, if a sufiiciently high constant temperature is maintained, an increase in pressure will give an increasing yield of iron precipitate up to 90 per cent or more of the iron originally present in the treated liquor.

The foregoing relationship of temperature and/or pressure is true for any sulfate liquor, composed substantially of iron and sulfuric acid, where the free sulfuric acid content of the treated liquor isnot greater than 40 per cent by weight, and where the iron sulfate content is not less than 7 per cent. Working within the concentration limits given, the yield of iron precipitate will of course vary for different conditions of concentration. This fact has been previously explained. It is true, however, that for a liquor of any concentration within the limits given, a yield of iron precipitate can be obtained at certain optimum conditions of temperature and pressure, and sub stantially per cent yields will result if the temperature and/ or pressure is raised sufficiently.

Between the threshold point A and point B in Fig. 1, iron is precipitated but slowly. But with a comparable increase in temperature or pressure, as indicated by points B and C, a remarkable increase in precipitation occurs.

The general curve shown in Fig. 2 is a composite of a number of curves formed by plotting time against per cent of dissolved iron precipitated.

The presence of a manganese salt in the liquor being treated is helpful. This seems to act as a catalyst to facilitate the precipitation of the basic ferric sulfate. Small amounts are efiective. In practice, I have used 0.24 per cent of manganese, as sulfate. Amounts down to 0.05 per cent may be used. Considerably larger amounts than 0.24 per cent may be employed without in any way adversely affecting the operation.

My invention is applicable to either batch pickling or continuous pickling operations. The autoclaving process would be particularly advantageous when used in conjunction with continuous strip pickling. In such an operation, pickle liquor can be Withdrawn from any part of the pickling unit, sent to the autoclave for precipitation of part of the iron, then, after separation of precipitated iron from solution recirculated to the pickling unit, where make-up acid may be added. By the above method of recirculation it is possible to maintain the pickle liquor in the pickling unit at practically a constant concentration at all times.

My processcan be used on sulfate liquors other than those from pickling operations, for instance, liquor which has been obtained from treating titania ores with sulfuric acid, and which contain iron in solution.

It is to be understood that my invention is not limited to the specific examples given herein, and that various modifications thereof may be employed without departing from the spirit or scope of my invention.

I claim: 7

l. A process of regenerating spent sulfate liquor containing not less than 10 per cent by weight of iron sulfate and between 5 and 10 per cent by weight of free sulfuric acid which comprises treating said liquor in the presence of an oxidizing agent of the group comprising air, oxygen and oxygen-enriched air at a total pressure not less than 450 p. s. i. and at a temperature of not less than 350 F. fora sufficient time to precipitate at least 60% of the iron as basic ferric sulfate, then separating the said precipitate from the liquor.

2. A process of regenerating spent sulfate liquor containing not less than-10 per cent by weight of iron sulfate and between 10 and 20% by weight of free sulfuric acid which comprises treating said liquor in the presence of an oxidizing agent of the group comprising air, oxygen and oxygen-enriched air at a total pressure not less than 450 p. s. i. and ata temperature of not less than 380 F. for a sufficient time to precipitate at least 60% of the iron as basic ferric sulfate, then separating the said precipitate from the liquor.

3. A process of regenerating spent sulfate liquor containing not less than 10 per cent by weight of iron sulfate and between 20 and 40 per cent by weight of free sulfuric acid which comprises treating said liquor in the present of an oxidizing agent of the group comprising air, oxygenand oxygen-enriched air at a total pressure not less than 480 p. s. i. and at a temperature of not less than 400 F. for a sufiicient time to precipitate at least 60% of the iron as basic ferric sulfate, then separating the said precipitate from the liquor.

4. A process of regenerating spent sulfate liquor containing not less than 10 per cent by weight of iron sulfate and between 5 and ,40 per cent by weight of free sulfuric acid which comprises treating said liquor in'the presence of an oxidizing agent containing gaseous oxygen in an amount at least twice the amount of the oxygen contained in air at a total pressure not less than 350 p. s. i. and at a temperature of not less than 385 F. for

a sufiicient time to precipitate at least 60% of the iron 5 as basic ferric sulfate, then separating the said precipitate from the liquor.

References Cited in the file of this patent UNITED STATES PATENTS McKechnie Jan. 10, 191 1 McKechnie Dec. 17, 1912 Clark July 29, 1924 Clark Sept. 22, 1942 

1. A PROCESS OF REGENERATING SPENT SULFATE LIQUOR CONTAINING NOT LESS THAN 10 PER CENT BY WEIGHT OF IRON SULFATE AND BETWEEN 5 AND 10 PER CENT BY WEIGHT OF FREE SULFURIC ACID WHICH COMPRISES TREATING SAID LIQUOR IN THE PRESENCE OF AN OXIDIZING AGENT OF THE GROUP CONPRISING AIR, OXYGEN AND OXYGEN-ENRICHED AIR AT A TOTAL PRESSURE NOT LESS THAN 450 P.S.I. AND AT A TEMPERATURE OF NOT LESS THAN 350* F. FOR A SUFFICIENT TIME TO PRECIPATE AT LEAST 60% OF THE IRON AS BASIC FERRIC SULFATE, THAN SEPARATING THE SAID PRECIPITATE FROM THE LIQUOR. 